It's an awkward waddle down to the panga, where Dario, the boatman, will drop us near the shore. My mask keeps fogging up and I start to panic that I won't be able to see underwater. 'Give it to Dario. He has the best spit in the Galapagos,' says Nelson. With a Windolene-clean mask I roll back and follow him into the blue.

A strong current carries us along like Pooh-sticks in a stream, over barnacled boulders where green moray eels lie slack-jawed like abandoned hosepipes. A swirling vortex of silver jacks glints above us. Two green turtles glide past lazily, accompanied by a crowd of hangers-on waiting for titbits to drop from their beaks. I'm absorbed by the antics of a family of giant parrotfish crunching the coral when I spot Nelson making the signal for hammerheads, as he bangs his fists against his temples. I spin around but see nothing, and so he beckons me out into the open ocean. Without the reef below, it feels like flying in an electric-blue vacuum. Shafts of sunlight from the surface above catching the plankton like the swirling dust in an empty church hall.

Moving rhythmically in unison, 50 hammerhead sharks or more slowly materialise below my ungainly flippers, rising up from the deep. I hang above them, too freaked to breathe, yet simultaneously spellbound. They come closer and closer, until the flexing muscles of their flanks shine as they glide by in formation. By the time they've cruised past, I've consumed nearly all my air. (As you can imagine, close shark encounters tend to accelerate oxygen intake.) Bobbing, gasping, to the surface, we find the air is full of the raucous cries of gulls. Dario hauls me into the dinghy like a slippery whale.

'See any sharks?'

'Just a few.'

South to Darwin the next day, we clock up several more dives with a cast of obliging hammerheads, and an appearance by a handful of nosy Galapagos sharks. My hands are raw from wrestling with my wetsuit but my breathing has finally calmed to a point where I can remain down for nearly an hour at a time. Gradually I'm growing relaxed in the presence of these four-metre-long predators. That's when Walter suddenly swirls by, descending at speed, his yellow tank disappearing into the deep. Adrenaline levels surge as I sight the tail of a sea monster. A whale shark probably 15m long is cruising by, looking every bit like the world's largest fish. I'm too low on air to follow, but the others have all managed to hitch a ride as it moves through the ocean in search of its never-ending supper.

After that high, there's nothing for it but to return to dry land, where we've a date with some of the non-aquatic creatures that have put these islands on the map. Afternoon excursions take in uninhabited islands such as North Seymour. Here carefully managed trails steer you past shoulder-high Palo Santo bushes bristling with nesting Frigate birds, characterised by their balloon-like scarlet necks (irresistible to females, apparently). They seem utterly unfazed by a band of ooh-ing humans centimetres from their nests, as do all the creatures here. It's a carnival of blue-footed boobies doing Mr Bean impressions with their goofy dance, part of a lengthy mating ritual. Coal-black marine iguanas blend perfectly with the dark lava rocks, spitting salt when anyone comes too close. Sea lions pose and preen for cameras like stars at a premiere, and neon-bright Sally Lightfoot crabs scuttle over toes. All that's missing are those textbook giant tortoises. Sadly, as the population dwindles, sightings are restricted to certain islands, including San Cristobal, where a breeding centre is successfully reintroducing them into the wild.

'Show me your boobies and I'll show you mine', grins Hamish as we compare snaps on our digital cameras. But even the boobies, with their feet the colour of Listerine, are left in the shade by the bizarre spectacle of miniature penguins waddling up the white sand beach at Bartolomé. Snorkelled-up, we leap in, but trying to keep abreast is a challenge - they can reach speeds of 50kph in the water.

No one bats an eyelid at our shark tales in the ramshackle bars of Puerto Ayora on sleepy Santa Cruz, where we celebrate our last night. They've heard it all before. At liberty at last to drink, we toast a week that's left us with limbs intact. I am glad I haven't had to call upon my dismemberment insurance - but I'm starting to wonder whether it covers missed flights too.

TRAVEL BRIEF

GETTING THERE

KLM (0870 507 4074, www.klm.com) flies from Heathrow to Quito, via Amsterdam, from £635. American Airlines (0845 778 9789, www.americanairlines.co.uk) flies via Miami, from £588. Aerogal (00 593 2294 2800, www.aerogal.com.ec) flies from Quito to San Cristobal and Baltra from £96.

TOUR OPERATORS

Scuba Safaris (01342 851196, www.scuba-safaris.com) has nine nights from £2,775pp, including flights from Heathrow, two nights' room only at the Hilton hotel in Guayaquil and seven nights on the Aggressor I or II, including all meals and drinks (except dinner on the final night of the cruise), five-and-a-half days of diving, tanks, weights and airfills. Or try Journey Latin America (020 8747 8315, www.journeylatinamerica.co.uk).

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